Manchester United could face UEFA sanctions after tens of flares were lit in the away end at Old Trafford during their Europa League clash with Saint-Etienne on Thursday night.
Supporters of the French club managed to smuggle pyrotechnics into the ground for their 3-0 Europa League defeat.
Smoke billowed from their section immediately before kick-off, with two flares thrown towards the playing surface.
Tens of flares slipped through the tightened security measures in place at Old Trafford
UEFA take such incidents seriously and usually issue charges. Ultimately, it is United's responsibility to make sure no flares enter the stadium.
'Host clubs and national associations are responsible for order and security both inside and around the stadium before, during and after matches,' UEFA rules read.
One club did, however, escape a charge after a recent episode, while it is understood United will discuss the matter at a post-match debrief.
Zlatan Ibrahimovic scored a hat-trick at Old Trafford as Manchester United won 3-0
The fact that so many flares were let off came as a surprise given United's strict security policy outside the ground.
Supporters are searched as they enter but flares were not spotted by staff.
United have upped their measures in a direct response to the Stade de France terror attacks in 2015 and, as Sportsmail revealed last month, now have a counter terrorism officer.